Cryopumps cooled by two stage closed cycle coolers are used to create a vacuum in a work chamber. When cryopumps are used to create a vacuum in a sputtering system where the process is carried out in an argon, oxygen or nitrogen environment, a common problem is "argon hang up". "Argon hang up" occurs when a valve between the work chamber and the cryopump is opened to expose the very high vacuum cryopump to a lower vacuum work chamber. To achieve a work chamber vacuum pressure of 10.sup.-7 torr, argon gas must be condensed on the cold second stage array at a temperature of 10 to 20K. If any argon gas pumps on the first stage array, sublimation from the frontal array causes the pressure in the system to "hang up" at a higher pressure.
A problem related to "argon hang up" can occur as a result of condensation of gases on the side of the second stage refrigerator cylinder. This problem is particularly apparent where an open second stage array is used to provide for maximum flow to an adsorbent material on the back side of the array. At normal operating temperatures, there is a temperature gradient along the length of the refrigerator cylinder from the approximately 77K first stage heat sink to the 15K second stage heat sink. Argon and other gases can condense along a zone of the refrigerator cylinder which is at a temperature of less than 50K. The temperature of that zone is determined by the system pressure. When a thermal load is applied to the first stage, as by opening a valve in the system, the first stage temperature increases and shifts the 50K zone along the length of refrigerator cylinder. As that zone shifts, gas which had been frozen out on the cylinder is rapidly liberated. That rapid evaporation results in a sharp increase in the work chamber pressure. Further, even when the thermal load on the first stage is constant, a displacer within the refrigerator cylinder reciprocates and causes continuous movement of the critical zone. That movement of the critical zone results in a high frequency fluctuation of the pressure in the work chamber.
U.S. Pat. No. 4,546,613 to Eacobacci et al. presents solutions to hangup. To avoid the problems caused by condensation of argon and other gases on the second stage refrigerator, a close fitting sleeve or shield surrounds the refrigerator cylinder. That sleeve or shield is in thermal contact with the second stage heat sink but is not in contact with the refrigerator cylinder. Most gas which passes the second stage array is condensed on the shield before it reaches the cylinder. A narrow gap of about 0.1 inch or less between the shield and warmer first stage array surfaces prevents gas from accessing the cylinder surfaces due to condensation in the gap. The gap is ideally controlled to a 1/w.gtoreq.5 in order to insure multiple gas surface collisions and therefore maximize condensation on the shield. With the shield held at the low temperature of the second stage heat sink, gas which condenses on the shield is held there and does not subsequently evaporate with displacer motion or high heat load to the first stage.